For example, a half-bridge circuit, a full-bridge circuit, and a three phase bridge circuit have been known as a bridge circuit for driving a load. In U.S. Pat. No. 5,619,047 corresponding to JP-A-8-213639, only the inductance is connected to the anode of the diode that is connected in antiparallel with the IGBT.
Thus, the induced voltage in the inductance changes in proportion to the change dI/dt in the current flowing through the diode. The polarity of the induced voltage is negative until the diode current reaches the reverse current peak. Then, when the current direction changes, the polarity of the induced voltage changes so that the positive voltage can be applied to the control electrode of the diode. U.S. Pat. No. 5,619,047 further discloses that the bonding wire connecting the electrode of the diode to the lead of the module can provide the necessary inductance.
In US 2008/0265975 corresponding to JP-A-2008-278552, the external diode is connected in antiparallel with the vertical MOSFET having the body diode. The gate voltages of the Hi-MOSFET and the Lo-MOSFET are switched between the voltages Von (>0), Voff1 (=0), and Voff2 (<0).
In the second OFF condition, the voltage Voff2 is applied to the gates of the Hi-MOSFET and the Lo-MOSFET so that the forward voltage of the body diode can become Vf2. Thus, the current is less likely to flow to the body diode. As a result, the current flowing to the external diode is increased, and the reverse recovery characteristics are improved. Further, the second OFF condition prevents the gate voltage from instantaneously exceeding the threshold voltage at the time of the switching operation.
It may be difficult to obtain desired reverse recovery characteristics before and after turn-ON of a switching device by combining the techniques disclosed in U.S. Pat. No. 5,619,047 and US 2008/0265975.